1. Field of the Invention
This invention relates to methods of making new and improved diamond coatings bonded to substrates, in which the coatings are protected by post-deposition treatment to form lubricating constituents in situ.
2. Background of the Invention
Diamond, diamond-like carbon and diamond-like hydrocarbon coatings have been employed both to provide hard faces on engineered materials and as abrasive coatings on articles made from such materials. Typically such diamond films and/or particles are applied using some form of chemical vapor deposition (CVD) process. Such processes generally use thermal decomposition of a mixture of hydrogen and carbon compounds, preferably hydrocarbons, into diamond generating carbon atoms preferentially from the gas phase activated in such a way as to avoid substantially the deposition of graphitic carbon. The specific types of carbon compounds useful for CVD include C1-C4 saturated hydrocarbons such as methane, ethane, propane and butane; C1-C4 unsaturated hydrocarbons such as acetylene, ethylene, propylene and butylene; gases containing C and O such as carbon monoxide and carbon dioxide; aromatic compounds such as benzene, toluene, xylene, and the like; and organic compounds containing C, H, and at least one of oxygen and/or nitrogen such as methanol, ethanol, propanol, dimethyl ether, diethyl ether, methylamine, ethylamine, acetone, and similar materials (see U.S. Pat. No. 4,816,286). The concentration of carbon compounds in the hydrogen gas can vary from about 0.1% to about 5%, preferably from about 0.2% to 3%, and more preferably from about 0.5% to 2%. The resulting diamond film in such a deposition method is in the form of adherent individual crystallites or a layer-like agglomerates of crystallites substantially free from intercrystalline adhesion binder.
Such CVD processes are known to those skilled in the art, and ordinarily use some form of energy (for example, microwave radiation, as in U.S. Pat. No. 4,859,493 and in U.S. Pat. No. 4,434,188) to pyrolyze hydrocarbon gases such as methane at concentrations of about 1% to 2% in a low pressure (about 10 torr) hydrogen atmosphere, causing deposition of diamond or "diamond-like carbon" (a-C) or "diamond-like hydrocarbon" (a-C:H) particles or film on a nearby substrate. (Diamond and "diamond-like carbon" (a-C) coatings have an atomic hydrogen fraction of zero; for "diamond-like hydrocarbon" (a-C:H) coatings that fraction ranges from about 0.15 to about 0.6. Diamond coatings have atom number densities around 0.29 gram-atoms per cubic centimeter; "diamond-like carbon" (a-C) and "diamond-like hydrocarbon" (a-C:H) materials are characterized by atom number densities above 0.19 gram-atoms per cc.) It is also known to assist the CVD process using a variety of techniques including (1) pyrolysis by a hot tungsten filament intended to generate atomic hydrogen near the substrate (HFCVD); (2) supplying electrons by negatively biasing the filament as in electron-assisted chemical vapor deposition (EACVD); (3) creating a plasma using microwave energy or RF energy (PACVD; see U.S. Pat. Nos. 4,504,519 and 5,382,293); (4) using an argon ion beam to decompose the hydrocarbon feedstock, as in U.S. Pat. No. 4,490,229 and (5) using direct-current electrical discharge methods. See, generally, John C. Angus and Cliff C. Hayman, "Low-Pressure, Metastable Growth of Diamond and `Diamondlike` Phases,"Science, Aug. 19, 1988, at p. 913. The disclosures of the U.S. patent references cited above are incorporated by reference herein.
The ion beam deposition method typically involves producing carbon ions by heating a filament and accelerating carbon ions to selected energies for deposit on a substrate in a high vacuum environment. Ion beam systems use differential pumping and mass separation techniques to reduce the level of impurities in the carbon ion flow to the growing film.
The chemical vapor deposition and plasma enhanced chemical vapor deposition methods are similar in operation. Both methods use the dissociation of organic vapors (such as CH.sub.3 OH, C.sub.H H.sub.2, and CH.sub.3 OHCH.sub.3) to produce both carbon ions and neutral atoms of carbon for deposit on a substrate. Plasma enhanced methods are described in U.S. Pat. Nos. 5,382,293 and No. 5,403,399, the disclosures of which are incorporated by reference herein.
It is also known to apply polycrystalline diamond layers using sintering at simultaneous high pressures (50 kbar) and temperatures (1300.degree. C.) to create conditions under which the diamond phase is thermodynamically stable, as in U.S. Pat. No. 5,370,195. And liquid-phase diffusion metallizing techniques also have been suggested for bonding diamond to certain types of substrates, as in U.S. Pat. No. 5,392,982.
Synthetic diamond-coated articles have found a wide variety of uses. U.S. Pat. No. 4,960,643, for example, discloses articles coated with synthetic diamond particles of controlled size, to which an overlying film, for example of chromium, has been applied to help the diamond layer resist scratching and wear. Other patents disclose various diamond-coated articles of manufacture, including bearings (U.S. Pat. No. 5,284,394); fasteners (U.S. Pat. No. 5,096,352); engine parts (U.S. Pat. Nos. 5,132,587 and 4,974,498) and the like.
It is known that the durability and frictional properties of diamond-coated engineered materials can be improved by applying coatings such as chromium over the diamond film (see, e.g., U.S. Pat. Nos. 4,960,643; 5,346,719 and 5,224,969), and that excess non-diamond carbon mixed with diamond in a matrix can improve wear resistance, as disclosed in U.S. Pat. No. 5,158,148. In the past, however, such coatings or matrices have been applied to diamond substrates (such as diamond particles in drill bit inserts and the like) by a multi-step process involving MVD or CVD creation of metal or carbide films on the surface of the diamond particles or by adding excess carbon during high pressure sintering.